Special case handling for merged chroma blocks in intra block copy prediction mode

In a computer system that implements a media decoder, a method comprising:

• receiving encoded data;
• and
• decoding a picture using the encoded data, wherein the decoding includes intra block copy (“BC”) prediction for a set of adjacent units of the picture, including:
• reconstructing block vector (“BV”) values associated with the adjacent units:
• for each unit of the set of adjacent units, performing the intra BC prediction for a primary component block of the unit using one of the BV values that is associated with the unit;
• deriving a BV value for secondary component blocks of the set of adjacent units based at least in part on one or more of the BV values associated with the adjacent units;
• and
• performing the intra BC prediction for the secondary component blocks using the derived BV value, wherein adjacent blocks among the secondary component blocks are merged for purposes of the intra BC prediction;
• wherein the decoding further includes special case handling for any value of the derived BV value referencing sample values outside a boundary, and wherein the reconstructing includes, for the special case handling, evaluating, according to a priority order, different candidate BV values used to determine the derived BV value until the derived BV value does not reference any sample values outside the boundary.

A computer system comprising a processor and memory, wherein the computer system is configured to implement a media decoder that performs media processing comprising:

• receiving encoded data;
• and
• decoding a picture using the encoded data, wherein the decoding includes intra block copy (“BC”) prediction for a set of adjacent units of the picture, including:
• for each unit of the set of adjacent units, performing the intra BC prediction for a primary component block of the unit using a block vector (“BV”) value associated with the unit;
• deriving a BV value for secondary component blocks of the set of adjacent units based at least in part on one or more of the BV values associated with the adjacent units;
• and
• performing the intra BC prediction for the secondary component blocks using the derived BV value, wherein adjacent blocks among the secondary component blocks are merged for purposes of the intra BC prediction;
• wherein the decoding further includes special case handling for any value of the derived BV value referencing sample values outside a boundary, the special case handling including padding sample values away from the boundary or using default sample values when the derived BV value references sample values outside the boundary.